package com.wc.codeforces.排序.Concatenation_of_Arrays;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;

/**
 * @Author congge
 * @Date 2024/10/20 17:35
 * @description https://codeforces.com/contest/2024/problem/C
 */
public class Main {
    /**
     * 思路：遵循小的往前放, 大的往后放
     */
    static FastReader sc = new FastReader();
    static PrintWriter out = new PrintWriter(System.out);
    static int N = 100010;
    static int[][] a = new int[N][2];
    static int n;

    public static void main(String[] args) {
        int T = sc.nextInt();
        while (T-- > 0) {
            n = sc.nextInt();
            for (int i = 0; i < n; i++) {
                a[i][0] = sc.nextInt();
                a[i][1] = sc.nextInt();
            }
            Arrays.sort(a, 0, n, (o1, o2) -> {
                if (Math.min(o1[0], o1[1]) == Math.min(o2[0], o2[1])) {
                    return Math.max(o1[0], o1[1]) - Math.max(o2[0], o2[1]);
                }
                return Math.min(o1[0], o1[1]) - Math.min(o2[0], o2[1]);
            });
            for (int i = 0; i < n; i++) {
                out.print(a[i][0] + " " + a[i][1] + " ");
            }
            out.println();
        }
        out.flush();
    }
}

class FastReader {
    StringTokenizer st;
    BufferedReader br;

    FastReader() {
        br = new BufferedReader(new InputStreamReader(System.in));
    }

    String next() {
        while (st == null || !st.hasMoreElements()) {
            try {
                st = new StringTokenizer(br.readLine());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return st.nextToken();
    }

    int nextInt() {
        return Integer.parseInt(next());
    }

    String nextLine() {
        String s = "";
        try {
            s = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return s;
    }

    long nextLong() {
        return Long.parseLong(next());
    }

    double nextDouble() {
        return Double.parseDouble(next());
    }

    // 是否由下一个
    boolean hasNext() {
        while (st == null || !st.hasMoreTokens()) {
            try {
                String line = br.readLine();
                if (line == null)
                    return false;
                st = new StringTokenizer(line);
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return true;
    }
}
